The present invention relates to counting apparatus and is particularly concerned with apparatus for counting the number of items crossing from one side of a given threshold to the other.
There are many applications in which it is desirable or necessary to count the number of items crossing a threshold. For example, in order to be able to comply with fire safety requirements, it is important to know the number of people present in a room or building and the invention will for convenience be described by reference to apparatus for performing this function. The apparatus of the invention is not however restricted to use in buildings, as it can be used for example to monitor crowds in railway stations or football grounds. Indeed, the apparatus of the invention need not even be used to count people as it could be used to count animals or even inanimate objects as they pass a given threshold.
It has been proposed to count people as they cross a threshold, such as a doorway, by viewing the entire threshold using a television camera and performing complicated image analysis to determine which parts of the image corresponding to individuals to be counted and the direction in which each individual is travelling across the threshold. The complexity of such a system is reflected in its cost and it is not economically viable in many, indeed most, applications.
A simpler system in use involves xe2x80x9cbeam breakingxe2x80x9d. A radiation beam, which may be of visible or infra-red light, is transmitted parallel to the threshold from one side and is either detected on the other side or reflected back by a mirror on the other side to a detector positioned on the same side as the transmitter. If a person crosses the threshold, the light beam is interrupted and the presence of the person can be sensed in this manner. If two such beams are used and spaced in the direction of travel across the threshold, a comparison of the signals from the two sensors will also indicate the direction in which a person has travelled, thereby allowing the people count to be incremented and decremented as people arrive and leave.
The problem with known beam breaking systems is that they cannot be relied upon when it is possible for more than one person to cross the threshold at the same time. This is because the systems cannot distinguish between a single person and two or more people walking side by side. Hence, beam breaking systems are usually used in conjunction with partitions that constrain people into walking in a single file past the sensors. However, partitioning an entrance and exit doorway in this manner is unacceptable as it restricts the rate at which people can pass through it and can constitute a serious hazard in the event of a fire.
The present invention seeks therefore to provide a counting apparatus that can be produced inexpensively but can count items as they cross a threshold that is wide enough to accommodate several items simultaneously.
According to the present invention, there is provided an apparatus for counting the number of items crossing from one side of a given threshold to the other, comprising a plurality of sensing means for mounting along the threshold and each capable of detecting an item passing through a section of the threshold, and processing means for comparing the detection signals of each pair of adjacent sensing means to prevent the same item from being counted separately by the adjacent sensing means, wherein the sensing means comprise an array of identical modules, each of the modules being connected to the adjacent modules and having an internal processor programmed to compare the detection signals generated by the module with the detection signals of the adjacent modules.
It is preferred for the sensing means to be mounted in use above the threshold and for each sensing means to detect when an item passes beneath it. However, the sensing means could be arranged below the threshold.
The apparatus of the invention can monitor a wide threshold because, instead of the entire threshold being monitored by the breaking of a single beam, each of the sensing means analyses a different section of the threshold. If two people should cross different sections of the threshold at the same time, then they will be detected by different sensing means and counted separately.
However, processing the signals from the different sensing means independently of one another would lead to counting errors, for reasons that will now be explained. If each section monitored by one sensor is only as wide as a person and the sections are contiguous a person crossing half way between two adjacent sensors would be detected by both sensors. This problem cannot be avoided by simply increasing the spacing between sensors as this would leave parts of the threshold unmonitored and it is aggravated by the fact that there is a wide variation in the size of different individuals.
A further source of counting errors arises on account of the fact that the direction of movement of people through the threshold should not be constrained by partitions. Consequently, the counting apparatus must be able to allow for a person entering the threshold through one section and leaving through another.
To avoid the same item being counted separately by two or more sensing means, in the present invention a detection of one person is carried out by analysing the outputs of a plurality of adjacent sensing means to ensure that the person is counted by one and only one of the sensing means.
The individual sensing means may comprise any form of sensors, for example acoustic or passive infra-red, but it is preferred that each sensing means should comprise a pair of infra-red sources spaced along a line perpendicular to the threshold and detecting means for detecting light reflected by a person passing beneath the infra-red sources, the detecting means being capable of distinguishing between the light emitted by the different sources.
Preferably, the infra-red sources transmit light of the same wavelength but at different times and the detecting means comprises a single detector for detecting light from both sources reflected by a person passing beneath the detector.
It is an important advantage of the apparatus of the invention that the use of a modular array avoids the need to design a special system for each installation and enables a threshold of any size to be monitored by linking together the appropriate number of modules.